Directional drilling is a technology of directing a wellbore along a predefined trajectory leading to a subsurface target. Directional drilling is an important technical means for sidetracking of an existing well, drilling multilateral wells, extended reach wells, or horizontal wells. Directional drilling can be employed to reach locations inaccessible to the drilling rig and to further develop oil and gas resources economically and effectively. Directional drilling can significantly improve the oil and gas production, reduce the cost and time of drilling operations. In addition it is conducive to protecting the environment. During the directional drilling process, a comprehensive knowledge of the wellbore direction and the drill bit orientation is essential to guarantee proper drilling procedure. Accurate attitude measurement of a downhole drilling tool not only ensures efficient drilling for predefined targets but also avoids collision with other wells in proximity. Thus, in addition to the conventional bottom hole assembly (e.g., including a drill bit, a positive displacement motor, a stabilizer, a drill collar, etc.), the directional drilling tool requires position sensors to measure the attitude of the downhole drilling tool, including an azimuth angle (a deviation angle from the north orientation in a horizontal plane), an inclination angle (a deviation angle from the vertical direction, also known as a pitch angle) and a toolface angle (equivalent to a roll angle in navigation).
With further exploration and development of oil fields, developing difficult-to-produce deposits, such as thin reservoirs, fault block reservoirs, marginal reservoirs and remaining reservoirs have been carried out, and requirements for accurate trajectory control are also increasing. In order to enlarge the contact area with oil and gas reservoirs in the wellbore, thereby increasing the production of oil wells, it is necessary to keep wellbore trajectory in the reservoir accurately. In order to obtain trajectory parameters near bit, a distance between downhole position sensors and the drill bit is important. An attitude measurement tool based on current MWD technology is installed behind a steerable tool, and attitude parameters including inclination, azimuth, toolface angles are measured 8 m to 20 m behind the drill bit, thus it is difficult to obtain a true wellbore position at the drill bit and determine the actual condition of entering the reservoir.
From the perspective of an attitude angle measurement principle, the existing MWD system includes two main technical lines, namely, a MWD system based on magnetic sensors and accelerometers as well as a MWD system based on gyroscopes and accelerometers.
(1) The MWD System Based on Magnetic Sensors and Accelerometers
An angle measurement unit of this type of MWD system consists of three orthogonal accelerometers and three orthogonal magnetic sensors. Quartz flexible accelerometers are generally selected as the accelerometers, and fluxgate magnetometers are generally selected as the magnetic sensors. Measurement results of the accelerometers are used to calculate the inclination angle and the toolface angle, and the azimuth angle is obtained using the inclination angle, the toolface angle and geomagnetic field data. As a result of reference to the magnetic north to measure an azimuth, the magnetometers require a clean environment without magnetic interference. A big problem faced by MWD systems based on magnetometers is magnetic interference, which mainly includes magnetic interference of a drill string itself and external magnetic interference generated by a surrounding environment. The magnetic interference of the drill string mainly affects the magnetometer along a direction of a rotation axis of the drilling tool, while the external magnetic interferences have effects on the three magnetometers. The external magnetic interference mainly comes from ferromagnetic casings of a producing well nearby, pyrite and other strata, solar storms, drilling fluid components, magnetic hot spots, and the like. The presence of the magnetic interference negatively affects azimuth measurement accuracy. A conventional MWD system is usually installed in the middle of a nearly 9-meter-long non-magnetic drill collar to isolate the magnetic interference of an upper drill string from the magnetic interference of a lower drill string as far as possible, but a near-bit attitude measuring apparatus needs to be installed immediately adjacent to the drill bit, and the magnetic interference generated by the drill bit and a steerable drilling tool is more serious than that of the conventional MWD system.
(2) The MWD System Based on Gyroscopes and Accelerometers
This type of MWD system uses gyroscopes to measure a change in an angular velocity along a sensitive axis of a sensor, which has been currently applied in the field of directional drilling. A gyroscope technology has an advantage in application scenarios where geomagnetic fields are shielded or magnetic interference is serious because the magnetic interference does not affect the performance of the gyroscope. At present, due to instrument sizes, robustness under vibration and shock condition, gyroscope measurement accuracy and other factors, the gyroscope technology is mainly applied in a wireline measurement system, and also has some restrictions in while drilling application scenarios. The maximum problem of the MWD system based on gyroscopes and accelerometers is that larger steady state errors may be introduced due to the larger output signal drift rate of an angular velocity gyroscope and the higher divergence angle of an integrated attitude angle. There are cumulative errors, greater cumulative errors especially in an underground high-temperature environment, since the output of the gyroscope increases with measurement time, and the volume and the reliability difficulty meet hostile operating conditions and a narrow space near the drill bit.
In summary, both the MWD system based on magnetic sensors and accelerometers and the MWD system based on gyroscopes and accelerometers have limitations in the application scenarios, especially for near-bit tool attitude measurement, and have difficulties in complex application environments such as downhole high temperature, severe vibration and shock, strong magnetic interference, and limited installation space.